1. Field of the Invention
The present invention relates to an extruding device for producing a honeycomb structure for use, for example, in an exhaust gas purifying device such as a catalytic converter for an automobile provided, for example, with an internal combustion engine.
2. Description of Related Art
A honeycomb structure is used for a catalytic converter in an automobile provided with an internal combustion engine. Such a honeycomb structure is, when it is constructed from a ceramic material, used as a catalyst carrier. In such a ceramic honeycomb structure, an integral construction has been proposed, which is constructed by a skin and cell walls for creating cells, wherein a thickness of the skin is larger than that of the cell wall, in order to obtain a desired mechanical strength.
In order to produce such a honeycomb structure, various techniques have been proposed such as those disclosed in Japanese examined patent publication No. 54-28850, Japanese examined patent publication No. 3-17644, and Japanese unexamined patent publication No. 7-52129. Japanese examined patent publication No. 54-28850 discloses an extruder apparatus for obtaining a honeycomb structure, which comprises an extrusion die formed with extrusion holes for extruding a honeycomb structure therefrom, the extrusion die having, at its bottom end surface, a peripheral portion inclined upwardly. The extruder apparatus further includes a control plate having an inclined surface faced with the inclined surface of the die. By this structure, an integrated honeycomb structure having a skin of a thickness more than four times the thickness of the cell wall can be obtained. In this prior art, a change in an outer diameter of the produced honeycomb structure, which is necessary for adjusting a variation in a shrinkage as generated during a firing of the product, is done by changing a shape of an adjusting plate, which is arranged at a location upstream from the feed passageways of the die or by changing the shape of the die itself. However, a change of such an adjustment plate requires a removal of the die due to the fact that the adjusting plate is located upstream from the die, which makes the working steps for an adjustment of the product diameter to be complicated.
In Japanese examined patent publication No. 3-17644, a die is proposed, which is, at its outer periphery, provided with a taper shaped secondary surface which inclined outwardly, and a mask having an outlet end which is tapered at the same angle as that of the tapered secondary surface, thereby preventing a skin of the product from collapsing. An adjustment of an outer diameter of the product for accommodating a variation in a shrinkage of the product during a firing is done by using a mask, so that a degree of the opening of a central plate in an annular back plate contacting with an inlet end of the mask is changed, i.e., the number of the feed holes to which the material is introduced is increased or reduced, so that an amount of flow of the material flowing into a gap between the secondary surface and the inlet surface of the mask is controlled. However, in this prior art, replacement of both the die and back plate is necessary every time a change in the outer diameter of the product becomes necessary. A temporary removal of the die is necessary in order to allow the back plate to be replaced due to the fact that the back plate is located upstream from the die, thereby causing the working steps for an adjustment of the product diameter to be complicated.
Japanese unexamined patent publication No. 7-52129 discloses a die having an outer tapered surface and a control plate facing the tapered surface of the die. In a space formed between the inclined surface and the control plate, the volume of the portion of the space located inwardly is smaller than that located outwardly. Due to this arrangement, an extrusion of a product, which has an uniform thickness of the skin so long as it is less than 5 times of the thickness of the cell walls, is possible. However, in this structure, an adjustment of an amount of flow of a material for changing an outer diameter of the product is, similar to the above mentioned prior arts, done by means of the back plate or masking plate which is located upstream from the die, which causes the working steps for changing the outer diameter to be difficult.
In Japanese Unexamined Patent Publication No. 7-60880, a die having, at its outer periphery, passageways for extruding portions of honeycomb structure corresponding to the cell walls and a flow control plate faced with an inclined surface of the die are disclosed. The surface of the die at the outlet ends of the passageways is outwardly inclined, thereby preventing the cell walls from being deformed at the outer periphery of the product, while obtaining an uniform thickness of the outer wall of the product. However, the prior art is defective in that the machining of the discharge passageways in the die is difficult. Furthermore, the arrangement of the back plate for adjustment of the flow amount of the material located upstream from the die causes the working to be complicated and difficult when the adjustment of the outer diameter of the product is required.